1. Field of the Invention
This invention relates to a sheet processing apparatus having a stapler function to staple sheets discharged from an image forming apparatus such as a copying machine, a facsimile, a printer, an offset printer, or the like, and more particularly to a sheet processing apparatus with an open/close switchable sheet discharging member for discharging a non-stapled sheet and stapled sheets.
2. Discussion of the Background
A sheet processing apparatus having a stapler function to staple sheets discharged from an image forming apparatus generally includes a function of stapling sheets and then discharging the stapled sheets, in addition to a function of discharging sheets without stapling. Further, such a sheet processing apparatus generally includes an output tray which receives sheets which are discharged after being stapled and the sheets discharged without being stapled as well.
FIG. 8 is a schematic drawing illustrating a configuration of a sheet processing apparatus disclosed, for example, in Japanese Patent Laid Open Publication No.5-305786. A sheet P conveyed from an image forming apparatus 500 is selectively guided either to a first conveying path 504 for stapling or a second conveying path 506 for discharging the sheet without stapling by an operation of a separation guide pick 502. The separation guide pick 502 operates in accordance with a selection between stapling and non-stapling modes.
In the stapling mode, the sheet P is guided to the first conveying path 504 and is stacked and aligned on a stack tray 508. Then, a plurality of the sheets P stacked in the stack tray 508 are stapled by a stapler (not shown) and the stapled sheets are pushed out by a sheets pushing up member (not shown) in the direction indicated by a broken arrow in the drawing and are discharged to an output tray 512 while being supported and sandwiched by a discharging member 510, provided at an exit of the sheet processing apparatus, for discharging the sheet therefrom. In the non-stapling mode, the sheet P is guided to the second conveying path 506 and discharged to the output tray 512 after being supported and sandwiched by the discharging member 510.
The discharging member 510 includes a pair of rollers, including a drive roller 514 and a driven roller 516. The driven roller 516 is selectively brought into and out of contact with the drive roller 514 in the direction indicated by an arrow. When the sheets P are started to be discharged (pushed upward) from the stack tray 508 in the stapling mode, the driven roller 516 moves apart from the drive roller 514 so that the discharging member 510 is in an open condition for discharging the sheets P without sandwiching them. The sheets P are started to be pushed upward through the discharging member 510 in this open condition. A discharging sensor (not shown) is provided adjacent to and upstream in the sheet conveying direction of the discharging member 510. When the discharging sensor detects the tail end of the sheets, the discharging member 510 changes to a closed condition with the driven roller 516 returned to the position to contact the drive roller 514. Thereafter, the sheets P are discharged to the output tray 512 being supported and sandwiched by the discharging member 510.
If the discharging member 510 supports and sandwiches the sheets P at an earlier timing, in other words if the discharging member 510 supports and sandwiches the sheets P from the tip portion of the sheets P in the conveying direction, the following problem arises. As illustrated in FIG. 9(a), when a curl of a stapled stack of sheets P is large, there exists a deviation of placements between the top and bottom sheets in the sheet conveying direction. Because the sheets P are stapled at the tail end parts thereof, the top side sheets of the stapled stack of sheets P gradually bulge resulting from being sandwiched and pressed by the rollers 514 and 516. As illustrated in FIG. 9(b), a bulge E is transferred toward the tail end parts of the sheets as the bulge E is pressed by the rollers 514 and 516, but the bulge E cannot be eliminated because the sheets P are stapled at the tail end parts of the sheets. The bulge E is pressed by the rollers 514 and 516 finally. This makes the sheets P thicken in the vicinity of the tail end of the sheets and as a consequence the sheets may not be discharged smoothly through the rollers 514 and 516. Though the above-mentioned problem is not described in the Japanese Patent Laid Open Publication No.5-305786, it seems that the problem does not exist in the sheet processing apparatus of the above publication because the sheets are started to be supported and sandwiched by the rollers after the discharging sensor provided in the vicinity of the discharging member 510 detects the tail end of the sheets.
However, in the sheet processing apparatus disclosed in the Japanese Patent Laid Open Publication No.5-305786, the discharging member 510 changes to the open condition from the closed condition when the stapled sheets are started to be discharged (pushed out) from the stack tray 508. Therefore, in order to stack and align sheets of every size in the conveying direction for stapling in the stack tray 508, a space for accommodating the biggest sized sheet is necessary between the discharging member 510 and the bottom part of the stack tray 508. Therefore, the apparatus is hard to be made compact, irrespective of the presence or absence of the non-stapling mode. In addition, the first conveying path for stapling and the second conveying path for non-stapling are located apart from each other with an open space existing between them, further making it harder to realize a compact design.